Methods For The Production Of Silver Nanocubes

ABSTRACT

Embodiments of the present invention are directed to novel methods for the solution-based production of silver nanocubes. This method differs, inter alia, from the traditional polyol method in that reactants (such as AgNO 3  and/or PVP) can be added to the reaction mixture in solid form rather than as a solution.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/230,949 filed on Aug. 3, 2009, incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made, in part, with government support under grant number(s) 0839504 from the National Science Foundation. The United States government may have certain rights to this invention.

The section headings used herein are for organizational purposes only and should not be construed as limiting the subject matter described in any way.

BRIEF DESCRIPTION OF DRAWINGS

The skilled artisan will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the present teaching in any way.

In the drawings, the sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles may not be drawn to scale, and some of these elements may be arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn may not be intended to convey any information regarding the actual shape of the particular elements, and may have been selected solely for ease of recognition in the drawings.

FIG. 1 is a transmission electron micrograph (TEM) image of a representative preparation of silver nanocubes wherein the image contains an inset of a section of the image that has been enlarged to show detail of the nanocubes.

All literature and similar materials cited in this application, including but not limited to patents, patent applications, articles, books and treatises, regardless of the format of such literature or similar material, are expressly incorporated by reference herein in their entirety for any and all purposes.

DESCRIPTION 1. Field

The present invention pertains to the field of nanotechnology and to the polyol mediated production of silver nanocubes.

2. Introduction

Production of silver nanostructures by the so called “polyol process” or “polyol method” is known (See for Example: Wiley et al., Shape-Controlled Synthesis of Metal Nanostructures The case of Silver, Chem. Eur. J., 11: 454-463 (2005) and Wiley et al., Polyol Synthesis of Silver Nanoparticles: Use of Chloride and Oxygen to Promote the Formation of Single-Crystal, Truncated Cubes and Tetrahedrons, Nano Letters, 4(9): 1733-1739 (2004)). In general, the polyol process is a solution-based method. According to early literature accounts, a silver ion containing solution is created by mixing a silver compound in a polyol solvent. The silver compound can be an inorganic salt such as silver nitrate (AgNO₃) or an organic salt such as silver acetate. The silver compound is reduced to silver metal by practice of the polyol process. It is this silver metal that forms the silver nanostructures (See: Ducamp-Sanguesa et al., Synthesis and Characterization of Fine and Monodisperse Silver Particles of Uniform Shape, Journal of Solid State Chemistry, 100: 272-280 (1992)). In a typical polyol synthesis, silver atoms (which produce the metal that forms the nanostructures) may be obtained by reducing AgNO₃ with ethylene glycol (EG) through the following reactions:

2HOCH₂CH₂OH→2CH₃CHO+2H₂O  (1)

2Ag⁺+2CH₃CHO→CH₃CHO—OHCCH₃+2Ag+2H⁺  (2)

In general, the polyol serves as a solvent for the silver compound as well as a solvent for the reaction. According to the early literature, the polyol is also the reducing agent that reduces the silver compound to silver metal (See: Fievet et al., Homogeneous and Heterogeneous Nucleations In The Polyol Process For The Preparation of Micron And Submicron Size Metal Particles, Solid State Ionics, 32-33: 198-205 (1989) at pages 198-199). Silver production is controlled by the rate of Ag(I) reduction, which increases with temperature (Ducamp-Sanguesa et al., Journal of Solid State Chemistry, 100: 272-280 (1992) at page 274, col. 2). Thus, the polyol process is typically practiced at elevated temperature although the reaction has been known to occur at ambient temperature in the presence of poly(vinyl pyrrolidone) (PVP) (See: Carotenuto et al., Eur. Phys. J. B, 16: 11-17 (2000) at page 12, col. 2 and US Published Patent Application No. US 2007/0034052 A1 to Vanheusden et al. published on Feb. 15, 2007 at paragraph 38).

In early experiments the polyol process was practiced by either combining (e.g. mixing) the silver compound with polyol and heating (See: Figlarz et al., U.S. Pat. No. 4,539,041) or dissolving the silver compound in the polyol and mixing the solution with a portion of heated polyol (Ducamp-Sanguesa et al., Journal of Solid State Chemistry, 100: 272-280 (1992)). In either case, the combined reactants were heated until the metal formed (usually as a metallic precipitate or powder). The metal was then typically isolated by filtration or decantation. These early experiments don't appear to discuss nanostructure production of any kind.

One problem observed with early practice of the polyol process was that the metal particles sintered (i.e. would aggregate to larger metal particle(s)). However, it was found that sintering in the polyol process could be minimized or prevented by use of an organic protective agent (Ducamp-Sanguesa et al., Journal of Solid State Chemistry, 100: 272-280 (1992) at page 275, col. 2). A commonly used organic protective agent is polyvinylpyrrolidone (PVP). Thus, the polyol process has typically thereafter been practiced by independently combining a solution of the silver compound (i.e. the ‘silver solution’) and a solution of the organic protective agent; often by combining these solutions with a heated portion of the polyol.

Some of the more recent literature reports have been directed to adaptations of the polyol process for the production of various selected nanostructures (See, for example: Sun et al., Shape-Controlled Synthesis of Gold and Silver Nanoparticles, Science, 298: 2176-2179 (2002); Sun et al., Polyol Synthesis of Uniform Silver Nanowires: A Plausible Growth Mechanism and the Supporting Evidence, Nano Letters, 3(7): 955-960 (2003); Wiley et al., Polyol Synthesis of Silver Nanoparticles: Use of Chloride and Oxygen to Promote the Formation of Single-Crystal, Truncated Cubes and Tetrahedrons, Nano Letters, 4(9): 1733-1739 (2004); Wiley et al., Polyol Synthesis of Silver Nanostructures: Control of Product Morphology with Fe(II) or Fe(III) Species, Langmuir, 21(18): 8077-8080 (2005); Wiley et al., Shape-Controlled Synthesis of Metal Nanostructures: The Case of Silver, Chem. Eur. J., 11: 454-463 (2005); Wiley et al., Maneuvering the Surface Plasmon Resonance of Silver Nanostructures through Shape-Controlled Synthesis, J. Phys. Chem. B., 110: 15666-15675 (2006) and Wiley et al., Synthesis of Silver Nanostructures with Controlled Shapes and Properties, Acc. Chem. Res., 40: 1067-1076 (2007)).

The polyol process can be practiced with addition of a seeding material to promote silver nanostructure formation in a so-called “seeding process”. Alternatively, in a “seedless/self-seeding process” silver metal clusters produced during the reaction serve as the seed for silver nanostructure formation. When a seeding process is used, the reactions have been characterized as either heterogeneous or homogeneous. In a homogeneous reaction, seeds are first formed using the metal from which the nanostructures are created. For example, if silver nanostructures are the desired product, the seeds are silver. By comparison, in a heterogeneous reaction, seeds of a different metal are prepared and then the nanostructures of the desired metal are grown from these seeds. For example, if silver nanostructures are desired in a heterogeneous reaction, platinum seeds could be prepared from which the silver nanostructures are then grown.

There are various other methods for the production of silver nanocubes that do not involve the polyol process. (See: Liu et al., Separation and Study of the Optical Properties of Silver Nanocubes by Capillary Electrophoresis, Chemistry Letters, 33(7): 902-903 (2004); Yu et al., Controlled Synthesis of Monodisperse Silver Nanocubes in Water, J. Am. Chem. Soc., 126: 13200-13201 (2004); Fan et al., Epitaxial Growth of Heterogeneous Metal Nanocrystals: From Gold Nano-octahedra to Palladium and Silver Nanocubes, J. Am. Chem. Soc., 130: 6949-6951 (2008); Kundu et al., Polyelectrolyte mediated scalable synthesis of highly stable silver nanocubes in less than a minute using microwave irradiation, Nanotechnology, 19: 065604 (2008) and Akhavan et al., Silver nanocube crystals on titanium nitride buffer layer, J. Phys. D: Appl. Phys., 42: 105305 (2009).

Silver nanocubes are electrically conductive. Nanocubes possess some properties that differ from other nanostructures (e.g. silver nanowires). It has been proposed that nanocubes may prove to be useful in pinning of the magnetic domains to attain ferromagnetic properties at the nanoscale and/or for self-assembly into highly packed structures that may be used as templates to make superlattices for high density storage applications (See: Kundu et al., Nanotechnology, 19: 065604 (2008) at page 1, col. 1). Hollow version of metal nanocubes have unique near-infrared properties that can be useful for imaging, heating, etc.

Because silver nanocubes (and hollow versions of silver nanocubes) are commercially desirable, it would be useful to have improved/different methods for their commercial manufacture. To date, it does not appear that anyone has prepared silver nanocubes by addition of solid silver compound (rather than as a silver compound containing solution) to other reactants according to the polyol process.

3. Definitions

For the purposes of interpreting this specification, the following definitions will apply and whenever appropriate, terms used in the singular will also include the plural and vice versa. In the event that any definition set forth below conflicts with the usage of that word in any other document, the definition set forth below shall always control for purposes of interpreting the scope and intent of this specification and its associated claims.

Notwithstanding the foregoing, the scope and meaning of any document incorporated herein by reference should not be altered by the definition presented below. Rather, said incorporated document should be interpreted as it would be by the ordinary practitioner based on its content and disclosure and then interpreted with respect to how it relates to the content of the description provided herein.

The use of “or” means “and/or” unless stated otherwise or where the use of “and/or” is clearly inappropriate. The use of “a” means “one or more” unless stated otherwise or where the use of “one or more” is clearly inappropriate. The use of “comprise”, “comprises”, “comprising”, “include”, “includes”, and “including” are interchangeable and not intended to be limiting. Furthermore, where the description of one or more embodiments uses the term “comprising”, those skilled in the art would understand that in some specific instances, the embodiment or embodiments can be alternatively described using language “consisting essentially of” and/or “consisting of”.

As used herein, ‘acid compound’ refers to any compound having a pK_(a) lower than 7. In some embodiments, the acid compound has a pK_(a) less than 3.5. In some embodiments, the acid compound has a pK_(a) less than 2. In some embodiments, the acid compound has a pK_(a) less than 1. The acid compound can be any acid that does not appreciably interfere with the reduction of the silver compound to silver metal or otherwise interfere with the reaction. In some embodiments, the acid compound may also be selected to avoid halide ion or iron. In some embodiments, the acid compound is intended to refer to a mixture of two or more compounds that have a pK_(a) less than 7, less than 3.5, less than 2 or less than 1. For the avoidance of any doubt however, the ‘acid compound’ is not intended to refer to a ‘silver compound’ as discussed and defined below.

With respect to an individual nanostructure, ‘aspect ratio’, as used herein, refers to the length divided by diameter of the individual nanostructure. For example, a nanowire having a length of 30,000 nanometers (30 μm) and a diameter of 50 nanometers would have an aspect ratio of 600 (30,000/50=600).

As used herein with respect to practice of the methods, the terms ‘added’, ‘mixed’ or ‘combined’ are generally interchangeable and refer to the act of adding, mixing or combining one or more of the reactants with one or more other reactants. This can occur by adding reactants to, or mixing or combining the reactants in, the reaction vessel and/or with each other.

As used herein, ‘halide ion’ refers to fluoride ion, chloride ion, bromide ion or iodide ion.

As used herein ‘nanocube’ refers to a nanostructure having a shape that is roughly cubic (i.e. has approximately the same height, width and depth dimensions, wherein no side is greater than 2-4 times larger than another side.). The corners of the cubic structure may be pointed or slightly rounded. The nanocube could subsequently be hollowed to created a nanocube shelled material.

As used herein ‘nanostructure’ refers to any of the various structures of the metal formed in the polyol process wherein one of the dimensions is less than 1000 nm. In some preferred embodiments, the smallest dimension is less than 500 nm, is less than 250 nm or even less than 100 nm. Examples of nanostructures include, but are not limited to, nanoparticles, nanopyramids, nanorods, nanocubes, nanobeams, nanobelts, nanoplates, nanocables and nanowires.

As used herein, ‘nanowire’ refers to a nanostructure wherein the length and diameter dimension produce aspect ratios of at least 10.

As used herein, ‘weight percent’ or ‘wgt %’ refers to percent by weight or mass. For example, the phrase ‘at least 10 weight percent (10 wgt %) silver nanocubes as compared with a weight percent of all other silver nanostructures in the product solution’ refers to calculating, as a percentage, a fraction represented by the weight or mass of all silver nanocubes present in the product solution divided by the weight or mass of all other silver nanostructures present in the product solution. Although real-time monitoring of the reaction may permit this analysis (Wiley et al., Synthesis of Silver Nanostructures with Controlled Shapes and Properties, Acc. Chem. Res., 40: 1067-1076 (2007)), in practice this can also (more practically) be determined by calculating (determining) the weight of silver nanocubes isolated from the reaction divided by the theoretical weight of all silver metal that could be produced (and thereby exist as silver nanostructures) by the reaction.

As used herein, ‘product solution’ refers to the solution comprising the entirety of all the reactants before any dilution or concentration is performed.

As used herein, ‘reactant’ refers to a compound, or solution comprising a compound, that reacts in the reaction or that is capable of influencing the abundance of nanocubes (or nanostructures) formed. In embodiments of this invention, reactants include, but are not necessarily limited to, the polyol (or polyols), the silver compound(s), the organic protective agent(s), an acid compound (or compounds), the compound(s) that comprise halide ion (e.g. NaCl, NaBr or NaI) and/or the compound(s) that comprise Fe(II) and/or Fe(III) either as solids, liquids, gases or in solution. In some embodiments, Na₂S is a reactant.

As used herein ‘reaction temperature’ refers to the temperature of the heat source applied to the reaction vessel or the actual temperature of the reaction mixture during the reaction as determined by direct monitoring. For example, the reaction temperature can be the temperature of an oil bath used to heat the vessel containing all the reactants of a polyol reaction or could be the temperature of the reaction mixture as determined by a thermometer or thermocouple inserted into said reaction mixture. Typically the reaction temperature of a reaction is monitored by one or the other of these two methods.

As used herein, ‘reaction mixture’ refers to both the mixture of reactants as fully combined as well as to a mixture to which one or more of the reactants is being added but to which at least a portion of all the reactants has been added such that the reaction can begin. For example, in the polyol process, it is common to add dropwise the silver solution and a solution comprising the organic protective agent into a vessel comprising polyol. From the time the first drops of silver solution and solution comprising the protective agent mix with the polyol in the vessel, the reaction has begun despite the fact that not all of each of the reactants has yet been combined. Thus, according to this definition, the vessel comprising the drops of silver solution, solution comprising the organic protective agent and the polyol is a reaction mixture.

4. General

It is to be understood that the discussion set forth below in this “General” section can pertain to some, or to all, of the various embodiments of the invention described herein.

Polyol(s)

The polyol is selected to be capable of reducing the silver compound to silver metal at the reaction temperature when present in the reaction mixture. The polyol can also be selected for its ability to dissolve the silver compound. The polyol can also be selected based upon its ability to influence the formation of silver nanocubes over other silver nanostructures under the reaction conditions. The polyol can also be selected for its ability to dissolve the organic protective agent as described infra. The foregoing criteria are not mutually exclusive such that, the polyol is typically selected based on a consideration of all of the foregoing criteria (and possibly other criteria).

The polyol may be a single polyol or a mixture of two or more polyols (e.g. three, four, five or more polyols). Whenever the term “polyol” is used herein, this term is meant to include both a single polyol and a mixture of two or more polyols unless used as part of the phrase “polyol or polyols” or “polyol(s)” (both of which include the singular and plural version of this term) or where use of the singular term is clearly intended or required.

The polyol may have any number of hydroxyl groups (but at least two) and carbon atoms provided that it comprises 2 or more hydroxyl groups. Also, the polyol may comprise heteroatoms (such as, e.g., O and N); not only in the form of hydroxyl groups, but also in the form of, e.g., ether, ester, amine and/or amide groups and the like (for example, the polyol may be a polyester polyol, a polyether polyol, a polyamide polyol, etc.). A polyol can be either an aliphatic glycol or a corresponding glycol polyester. Said aliphatic glycol, for instance, can be an alkylene glycol having up to 6 carbon atoms in the main chain. Examples include ethanediol, a propanediol, a butanediol, a pentanediol or a hexanediol, as well as polyalkylene glycols derived from these alkylene glycols.

In some embodiments, the polyol comprises from about 2 to about 6 hydroxy groups (e.g., 2, 3 or 4 hydroxy groups) and from 2 to about 12 carbon atoms (e.g., 3, 4, 5 or 6 carbon atoms). The (alkylene) polyol can be a glycol, i.e., compounds which comprise two hydroxyl groups bound to adjacent (aliphatic or cycloaliphatic) carbon atoms. For example, the glycols can comprise up to about 6 carbon atoms, e.g., 2, 3 or 4 carbon atoms. Some useful polyols include glycerol, trimethylolpropane, pentaerythritol, triethanolamine and trihydroxymethylaminomethane.

In some embodiments, a polyol can be ethylene glycol, diethylene glycol, tri-ethylene glycol, a propylene glycol, a butanediol, a dipropylene glycol or a polyethylene glycol that is liquid at the reaction temperature, such as for example, polyethylene glycol 300. Other useful polyols include tetra-ethylene glycol, propanediol-1,2, di-propylene glycol, butanediol-1,2, butanediol-1,3, butanediol-1,4 and butanediol-2,3. The use of these glycols is advantageous because of their significant reducing power, their boiling temperature of between 185° C. and 328° C., their proper thermal stability and their low cost price. Furthermore, these glycols raise few toxicity problems.

Another non-limiting grouping of polyols suitable for use in the process of the present invention includes: ethylene glycol, glycerol, glucose, diethylene glycol, tri-ethylene glycol, a propylene glycol, a butanediol, a dipropylene glycol and/or a polyethylene glycol.

Of course, it also is possible to use other polyols than those mentioned above, either alone or in combination. For example, sugars and sugar alcohols can form at least a part of the polyol reactant. While polyols that are solid or semi-solid at room temperature may be employed, the employed polyol or at least the employed mixture of polyols will generally be liquid at room temperature and at the reaction temperature, although this is not mandatory.

From an economic and environmental standpoint, it is interesting to note that the polyols can often be re-used. For example, the polyols can usually be recaptured and used again in other reactions or else they can be purified by distillation and/or crystallization prior to reuse.

According to embodiments of the present invention the polyol and the associated reaction conditions are selected to preferentially produce silver nanocubes as compared with other nanostructures. Thus, using no more than the guidance provided herein and routine experimentation, one of skill in the art will be able to select polyols that can be used (according to the presently disclosed inventive methods) to selectively produce silver nanocubes.

Silver Compound

The silver compound is the source of the silver metal that produces the silver nanostructures according to the polyol method. In general, the ‘silver compound’ can be any silver containing compound that produces silver metal when reduced. Any counterion (e.g. anion) should not interfere with the reduction reaction. The silver compound should be at least partially soluble in the polyol. Complete solubility is not required because suspensions and/or co-solvents can be used. Whenever the term ‘silver compound’ is used herein, this term is meant to include either a single silver compound or a mixture of two or more silver compounds unless use of the singular term is clearly intended or required.

According to the polyol method, the silver compound is reduced by the polyol (and/or by supplemental reducing agents) to thereby produce silver metal in-situ. The silver metal that is formed can produce various types of silver nanostructures depending on the conditions employed. According to various embodiments of the present invention, the silver compound, other reactants and the associated reaction conditions are selected to preferentially produce silver nanocubes as compared with other nanostructures wherein the silver compound is added to the reaction as a solid (rather than as a silver solution).

The silver compounds that may be used in various embodiments of the present invention include all silver compounds that a polyol (and/or any supplemental reducing agents) can reduce to the corresponding silver metal (oxidation state=zero). The silver compound is usually selected to be soluble to at least some extent in the polyol (other solvent or mixture of polyol and other solvent(s)) at room temperature and at the reaction temperature so that there is no substantial precipitation or other separation of the silver compound (as compared with the silver metal) from the liquid phase when the silver compound is combined with the other reactants; one or more of which may be at any suitable temperature (e.g. from room temperature to the reaction temperature or even above the reaction temperature).

So long as it does not preclude production of nanocubes, there is not any particular limitation on the amount of silver compound that can be used since, as the silver compound is consumed, it is converted to silver metal. Although not an absolute limitation, in practice the concentration of silver (either as an ion or as metal) in the product solution can be; in the range of about 0.1 mmolar to about 100 mmolar, in the range of about 1 mmolar to about 330 mmolar or in the range of about 10 mmolar to about 25 mmolar. In practice, these concentrations can be determined based upon moles of silver added to the reaction divided by the total volume of all reactants. These concentration ranges include concentrations that are far higher than have been used in other reported preparations.

In general, the silver compound can be a silver oxide, a silver hydroxide or a silver salt (organic or inorganic). Non-limiting examples of suitable silver compounds include silver salts of inorganic and organic acids such as, e.g., nitrates, nitrites, sulfates, halides (e.g., fluorides, chlorides, bromides and iodides), carbonates, phosphates, azides, borates (including fluoroborates, pyrazolylborates, etc.), sulfonates, carboxylates (such as, e.g., formates, acetates, propionates, oxalates and citrates), substituted carboxylates (including halogenocarboxylates such as, e.g., trifluoroacetates, hydroxycarboxylates, aminocarboxylates, etc.) and salts and acids wherein the silver is part of an anion (such as, e.g., hexachloroplatinates, tetrachloroaurate, tungstates and the corresponding acids) as well as combinations of any two or more of the foregoing. Further non-limiting examples of suitable silver compounds for the process of the present invention include alkoxides, complex compounds (e.g., complex salts) of silver such as, e.g., beta-diketonates (e.g., acetylacetonates), complexes with amines, N-heterocyclic compounds (e.g., pyrrole, aziridine, indole, piperidine, morpholine, pyridine, imidazole, piperazine, triazoles, and substituted derivatives thereof), aminoalcohols (e.g., ethanolamine, etc.), amino acids (e.g., glycine, etc.), amides (e.g., formamides, acetamides, etc.), and nitriles (e.g., acetonitrile, etc.) as well as combinations of any two or more of the foregoing. In some embodiments, the silver compound is selected such that the reduction by-product is volatile and/or can be decomposed into a volatile by-product at a relatively low temperature.

Another non-limiting grouping of silver compounds suitable for use in the process of the present invention includes: silver nitrate, silver nitrite, silver oxide, silver fluoride, silver hydrogen fluoride, silver carbonate, silver oxalate, silver azide, silver tetrafluoroborate, silver acetate, silver propionate, silver butanoate, silver ethylbutanoate, silver pivalate, silver cyclohexanebutanoate, silver ethylhexanoate, silver neodecanoate, silver decanoate, silver trifluoroacetate, silver pentafluoropropionate, silver heptafluorobutyrate, silver trichloroacetate, silver 6,6,7,7,8,8,8 heptafluoro-2,2-dimethyl-3,5-octanedionate, silver lactate, silver citrate, silver glycolate, silver glyconate, silver benzoate, silver salicylate, silver phenylacetate, silver nitrophenylacetate, silver dinitrophenylacetate, silver difluorophenylacetate, silver 2-fluoro-5-nitrobenzoate, silver acetylacetonate, silver hexafluoroacetylacetonate, silver trifluoroacetylacetonate, silver tosylate, silver triflate, silver trispyrazolylborate, silver tris(dimethylpyrazolyl)borate, silver beta-diketonate olefin complexes and silver cyclopentadienides as well as combinations of any two or more of the foregoing.

Another non-limiting grouping of silver compounds suitable for use in the process of the present invention includes: silver nitrate, silver nitrite, silver oxide, silver fluoride, silver hydrogen fluoride, silver carbonate, silver oxalate, silver azide, silver tetrafluoroborate, silver acetate, silver propionate, silver butanoate, silver ethylbutanoate or silver pivalate as well as combinations of any two or more of the foregoing.

In some embodiments, the silver compound is silver nitrate (AgNO₃). In some embodiments, the silver compound is silver acetate.

According to embodiments of the present invention the silver compound and the associated reaction conditions are selected to preferentially produce silver nanocubes as compared with other nanostructures. Thus, using no more than the guidance provided herein and routine experimentation, one of skill in the art will be able to select silver compounds (in appropriate amounts and/or concentrations (as appropriate)) that can be used to produce silver nanocubes.

Organic Protective Agent(s) (OPA)

The organic protective agent was originally introduced into the polyol mediated preparation of nanostructures to avoid particle sintering (See for example: Ducamp-Sanguesa et al. Journal of Solid State Chemistry, 100: 272-280 (1992)). More recent literature has suggested that particle morphology is at least in part dependent on the ratios of the organic protective agent (i.e. poly(vinyl pyrrolidone as used in the cited reference) to the silver compound (i.e. AgNO₃ as used in the cited reference) as well as the nature of the organic protective agent (See: Sun et al., Adv. Mater. 14(11): 833-837 (2002)). According to Vanheusden et al. (US 2007/0034052 A1), one of the other functions of the organic protective agent (referred to by them as the ‘Absorptive Substance’) is that it is capable of being absorbed onto the metal nanostructure. While Applicants do not intend to be bound to any theory as to how the organic protective agent acts in the polyol mediated synthesis of silver nanocubes, it is one of the reactants that is combined (e.g. mixed) according to various embodiments of the invention disclosed herein. Further features and characteristics of the organic protective agent and exemplary compounds are described in more detail infra.

Whenever the term ‘organic protective agent’ is used herein, this term is meant to include either a single organic protective agent or a mixture of two or more organic protective agents unless use of the singular term is clearly intended or required.

In the scientific and patent literature several other phrases have been used as a substitute for the term ‘organic protective agent’. Examples of alternative phrases include: ‘protective agent’ (See: Silvert et al., J. Mater. Chem., 6(4): 573-577 (1996), ‘coordination compound’ (See: Sun et al., Chem. Mater., 14: 4736-4745 (2002), ‘polymer capping agent’ polymeric capping agent', ‘capping agent’ or ‘capping reagent’ (See for example: Sun et al., Nano Letters, 3(7): 955-960 (2003), and ‘soft template’ (See: US 2007/0034052 A1). Use of the phrase ‘organic protective agent’ or ‘protective agent’ herein is intended to encompass all these other various phrases as well as other reactants known in the art that are added to the polyol synthesis of metal nanostructures to thereby reduce and/or prevent sintering (e.g. particle agglomeration).

In particular, it is thought that the organic protective agent acts to shield (e.g., sterically and/or through charge effects) the nanostructures and nanoparticles from each other to at least some extent and thereby reduce and/or prevent a direct contact between the individual nanostructures. In order to be effective, interaction between the organic protective agent and a nanostructure surface (e.g., the metal atoms on the surface of a nanoparticle) may manifest itself in an at least weak interaction between the organic protective agent and the surface of a nanostructure. In some embodiments, the interaction may be strong enough for the nanostructure-organic protective agent combination to withstand a washing operation with a solvent. In other words, merely washing the nanostructures with the solvent at room temperature is not likely to remove more than minor amounts (e.g., less than about 10%, less than about 5%, or less than about 1%) of that part of the organic protective agent that is in direct contact with (and perhaps weakly bonded to) the nanostructure surface. Of course, organic protective agent that is not in direct contact with a nanostructure surface and is merely associated with the bulk of the nanostructures as a contaminant (i.e., without any significant interaction with the nanostructures), can be removed from the nanostructures by washing the latter with a solvent. Further, in general the interaction between the organic protective agent and nanostructure should not be too strong and should be reversible under defined conditions. In addition to the organic protective agent's interaction with the silver metal, the organic protective agent may react with or reduce the silver compound during the reaction. This activity is not a problem as long as it does not interfere with the production of desired nanostructures, such as silver nanocubes.

In some embodiments, the organic protective agent can be a liquid. In such cases it may be combined (mixed) with other reactants directly or it may be first diluted by mixing with a solvent or mixture of solvents (referred to herein as the “OPA solvent”) that may or may not include the polyol. In some embodiments, the organic protective agent may be a solid which may, or may not, be dissolved in the OPA solvent before being combined (e.g. mixed) with the other reactants. It is not required that the organic protective agent dissolve in the OPA solvent. In some embodiments, a suspension of the organic protective agent can be used.

In some embodiments, the OPA solvent is the polyol or a mixture of the polyol and one or more other solvents. The one or more other solvents (that are not the polyol) may be selected to dissolve the organic protective agent, either alone or as a mixture (including mixtures with the polyol). There is no particular limitation on the OPA solvent so long as it does not inhibit or prevent the solution mediated production of desired nanostructures, such as silver nanocubes.

It is not a requirement that the organic protective agent be added to the reaction in solution as it may be added in liquid (if a liquid) or solid form (e.g. solid powder). However, as used herein ‘OPA solution’ refers to a solution comprising the organic protective agent is dissolved in a solvent and where the resulting solution is prepared for addition as a reactant. As suggested above, the organic protective agent may not always dissolve completely or even substantially such that the resulting solution is more accurately characterized as a suspension rather than a solution. As used herein, ‘OPA solution’ is intended to encompass suspensions as well as solutions where the organic protective agent is completely dissolved or essentially completely dissolved in the OPA solvent.

The organic protective agent is generally selected to not react with the polyol or any other solvent to any significant extent, even at elevated temperature (relative to ambient temperature). Any solvent used to dissolve the organic protective agent should also be selected so that it does not inhibit or prevent the solution mediated production of desired nanostructures, such as silver nanocubes. If the reaction mixture at the reaction temperature does not comprise any other solvent for the organic protective agent, the organic protective agent should also dissolve in the polyol to at least some extent.

The concentration of solutions of the organic protecting agent can be made to any concentration that is feasible and produces the desired nanostructures, such as silver nanocubes, in the reaction. Generally, the limitation on solution concentration is the solubility of the organic protective agent in the solvent (including mixture of solvents) selected. In some embodiments, the organic protective agent is added to the reaction as an OPA solution comprising the organic protective agent in an OPA solvent, wherein the concentration of the organic protective agent in the solution is: in the range of about 0.1 mmolar to about 100 mmolar. In some embodiments, the molar concentration of a solution of the organic protecting agent can be: in the range of about 0.1 molar to about 1.0 molar, in the range of about 0.5 mmolar to about 80 mmolar, in the range of about 1 mmolar to about 60 mmolar, in the range of about 5 mmolar to about 50 mmolar, or in the range of about 25 mmolar to about 40 mmolar.

It is to be understood that, for example, when a polymer like polyvinylpyrrolidone (PVP), having an average molecular weight of, for example, 55,000 is used as the organic protective agent, the concentration is calculated using the monomer weight and not the average molecular weight of the polymer. For example, the molar concentration of PVP solution would be calculated by dividing the grams of PVP used to make the OPA solution by 111 g/mole and not by 55,000 g/mole.

As discussed supra, the present invention also contemplates the use of two or more different types of organic protective agents being used together. For example, a mixture of two or more different low molecular weight compounds or a mixture of two or more of the same or different polymers may be used where if the same, the polymers are of different molecular weight. In some embodiments, a mixture of one or more low molecular weight compounds and one or more polymers can be used. For example, the organic protective agent can be a mixture of monomer and polymers (e.g. poly(vinyl pyrrolidone). In some embodiments, the organic protective agent can be a mixture comprising one or more commercially available polymer mixtures. For example, the organic protective agent may comprise poly(vinyl pyrrolidone) having an average molecular weight of 55,000 and poly(vinyl pyrrolidone) having an average molecular weight of 1,300,000). In some embodiments, the organic protective agent may comprise a mixture of different polymers. For example the organic protective agent may comprise poly(vinyl pyrrolidone) having an average molecular weight of 55,000 and poly(vinyl alcohol) having an average molecular weight of 35,000.

In some embodiments, the organic protective agent is a substance that is capable of electronically interacting with a metal atom of a nanoparticle. Such a substance can comprise one or more atoms (e.g., at least two atoms) with one or more free electron pairs such as, e.g., oxygen, nitrogen and sulfur. By way of non-limiting example, the organic protective agent may be capable of a dative interaction with a metal atom on the surface of a nanoparticle and/or of chelating the metal atom. For example, the organic protective agents can comprise one or two O and/or N atoms. The atoms with a free electron pair can be present in the substance in the form of a functional group such as, e.g., a hydroxy group, a carbonyl group, an ether group and an amino group, or as a constituent of a functional group that comprises one or more of these groups as a structural element thereof. Non-limiting examples of suitable functional groups include —COO—, —O—CO—O—, —CO—O—CO—, —C—O—C—, —CONR—, —NR—CO—O—, —NR¹—CO—NR²—, —CO—NR—CO—, —SO₂NR— and —SO₂—O—, wherein R, R¹ and R² each independently represent hydrogen or an organic radical (e.g., an aliphatic or aromatic, unsubstituted or substituted radical comprising from about 1 to about 20 carbon atoms). Such functional groups may comprise the above (and other) structural elements as part of a cyclic structure (e.g., in the form of a cyclic ester, amide, anhydride, imide, carbonate, urethane, urea, and the like). In some embodiments, the organic protective agent is, or comprises, a substance that is capable of reducing the metal compound. A specific, non-limiting example of such a substance is poly(vinyl pyrrolidone) (PVP). As such, PVP qualifies as a supplemental reducing agent as discussed infra under the heading ‘Supplemental Reducing Agents’.

In some embodiments, the organic protective agent can have a total of at least about 10 atoms per molecule which are selected from C, N and O, e.g., at least about 20 such atoms or at least about 50 such atoms. In some embodiments, the organic protective agent has a total of at least about 100 C, N and O atoms per molecule, e.g., at least about 200, at least about 300, or at least about 400 C, N and O atoms per molecule. In the case of polymers these numbers refer to the average per polymer molecule.

The organic protective agent may comprise a low molecular weight organic compound, not higher than about 500. In some embodiments, the organic protective agent may not be higher than about 300 molecular weight. In some embodiments, the organic protective agent may comprise an oligomeric or polymeric compound. By way of non-limiting example, poly(vinyl pyrrolidone) having an average molecular weight in the range of from about 10,000 to about 1,300,000, is particularly useful for preparing silver nanocubes. In some embodiments, poly(vinyl pyrrolidone) having an average molecular weight of 55,000 is selected.

Some non-limiting examples of organic protective agents that can be used alone or as mixtures include poly(vinyl pyrrolidone), poly(vinyl alcohol) and surfactants such as sodium dodecyl sulfate (SDS), laurylamine and hydroxypropyl cellulose.

Some non-limiting examples of the low molecular weight organic protective agent include fatty acids, such as, fatty acids having at least about 8 carbon atoms. Non-limiting examples of oligomers/polymers for use as the organic protective agent include homo- and copolymers (including polymers such as, e.g., random copolymers, block copolymers and graft copolymers) which comprise units of at least one monomer which comprises one or more O atoms and/or one or more N atoms. A non-limiting class of such polymers for use in the present invention is constituted by polymers which comprise at least one monomer unit which includes at least two atoms which are selected from O and N atoms. Corresponding monomer units may, for example, comprise at least one hydroxyl group, carbonyl group, ether linkage and/or amino group and/or one or more structural elements of formula: —COO—, —O—CO—O—, —CO—O—CO—, —C—O—C—, —CONR—, —NR—CO—O—, —NR¹—CO—NR²—, —CO—NR—CO—, —SO₂NR— and —SO₂—O—, wherein R, R¹ and R² each independently represent hydrogen or an organic radical (e.g., an aliphatic or aromatic, unsubstituted or substituted radical comprising from about 1 to about 20 carbon atoms).

Some non-limiting examples of corresponding polymers that can be the protective agent include polymers which comprise one or more units derived from the following groups of monomers:

(a) monoethylenically unsaturated carboxylic acids of from about 3 to about 8 carbon atoms and salts thereof. This group of monomers includes, for example, acrylic acid, methacrylic acid, dimethylacrylic acid, ethacrylic acid, maleic acid, citraconic acid, methylenemalonic acid, allylacetic acid, vinylacetic acid, crotonic acid, fumaric acid, mesaconic acid and itaconic acid. The monomers of group (a) can be used either in the form of the free carboxylic acids or in partially or completely neutralized form. For the neutralization alkali metal bases, alkaline earth metal bases, ammonia or amines, e.g., sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, magnesium oxide, calcium hydroxide, calcium oxide, ammonia, triethylamine, methanolamine, diethanolamine, triethanolamine, morpholine, diethylenetriamine or tetraethylenepentamine may, for example, be used;

(b) the esters, amides, anhydrides and nitriles of the carboxylic acids stated under (a) such as, e.g., methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl acrylate, hydroxyethyl acrylate, 2- or 3-hydroxypropyl acrylate, 2- or 4-hydroxybutyl acrylate, hydroxyethyl methacrylate, 2- or 3-hydroxypropyl methacrylate, hydroxyisobutyl acrylate, hydroxyisobutyl methacrylate, monomethyl maleate, dimethyl maleate, monoethyl maleate, diethyl maleate, maleic anhydride, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, acrylamide, methacrylamide; N,N-dimethylacrylamide, N-tert-butylacrylamide, acrylonitrile, methacrylonitrile, 2-dimethylaminoethyl acrylate, 2-dimethylaminoethyl methacrylate, 2-diethylaminoethyl acrylate, 2-diethylaminoethyl methacrylate and the salts of the last-mentioned monomers with carboxylic acids or mineral acids;

(c) acrylamidoglycolic acid, vinylsulfonic acid, allylsulfonic acid, methallylsulfonic acid, styrenesulfonic acid, 3-sulfopropyl acrylate, 3-sulfopropyl methacrylate and acrylamidomethylpropanesulfonic acid and monomers containing phosphonic acid groups, such as, e.g., vinyl phosphate, allyl phosphate and acrylamidomethylpropanephosphonic acid; and esters, amides and anhydrides of these acids;

(d) N-vinyllactams such as, e.g., N-vinylpyrrolidone, N-vinyl-2-piperidone and N-vinylcaprolactam;

(e) vinyl acetal, vinyl butyral, vinyl alcohol and ethers and esters thereof (such as, e.g., vinyl acetate, vinyl propionate and methylvinylether), allyl alcohol and ethers and esters thereof, N-vinylimidazole, N-vinyl-2-methylimidazoline, and the hydroxystyrenes.

Corresponding polymers may also contain additional monomer units, for example, units derived from monomers without functional group, halogenated monomers, aromatic monomers etc. Non-limiting examples of such monomers include olefins such as, e.g., ethylene, propylene, the butenes, pentenes, hexenes, octenes, decenes and dodecenes, styrene, vinyl chloride, vinylidene chloride, tetrafluoroethylene, etc. Further, the polymers for use as organic protective agent in the process of the present invention are not limited to addition polymers, but also comprise other types of polymers, for example, condensation polymers such as, e.g., polyesters, polyamides, polyurethanes and polyethers, as well as polysaccharides such as, e.g., starch, cellulose and derivatives thereof, etc.

In some embodiments, polymers for use in the present invention include polymers which comprise monomer units of one or more unsubstituted or substituted N-vinyllactams, such as those having from about 4 to about 8 ring members (e.g., N-vinylcaprolactam, N-vinyl-2 piperidone and N-vinylpyrrolidone. These polymers include homo- and copolymers. In the case of copolymers (including, for example, random, block and graft copolymers), the N-vinyllactam (e.g., N-vinylpyrrolidone) units can be present in an amount of at least about 10 mole-%, e.g., at least about 30 mole-%, at least about 50 mole-%, at least about 70 mole-%, at least about 80 mole-%, or at least about 90 mole-%. By way of non-limiting example, the comonomers may comprise one or more of those mentioned in the preceding paragraphs, including monomers without functional group (e.g., ethylene, propylene, styrene, etc.), halogenated monomers, etc.

If the vinyllactam (e.g., vinylpyrrolidone) monomers (or at least a part thereof) carry one or more substituents on the heterocyclic ring, non-limiting examples of such substituents may include alkyl groups (for example, alkyl groups having from 1 to about 12 carbon atoms, e.g., from 1 to about 6 carbon atoms such as, e.g., methyl, ethyl, propyl and butyl), alkoxy groups (for example, alkoxy groups having from 1 to about 12 carbon atoms, e.g., from 1 to about 6 carbon atoms such as, e.g., methoxy, ethoxy, propoxy and butoxy), halogen atoms (e.g., F, Cl and Br), hydroxy, carboxy and amino groups (e.g., dialkylamino groups such as dimethylamino and diethylamino) and any combinations of these substituents.

Non-limiting specific examples of vinyllactam polymers for use in the present invention include homo- and copolymers of vinylpyrrolidone which are commercially available from, e.g., International Specialty Products (www.ispcorp.com). In particular, these polymers may include:

(a) vinylpyrrolidone homopolymers such as, e.g., grades K-15 and K-30 with K-value ranges of from 13-19 and 26-35, respectively, corresponding to average molecular weights (determined by GPC/MALLS) of about 10,000 and about 67,000;

(b) alkylated polyvinylpyrrolidones such as, e.g., those commercially available under the trade mark GANEX® which are vinylpyrrolidone-alpha-olefin copolymers that contain most of the alpha-olefin (e.g., about 80% and more) grafted onto the pyrrolidone ring, mainly in the 3-position thereof; the alpha-olefins may comprise those having from about 4 to about 30 carbon atoms; the alphaolefin content of these copolymers may, for example, be from about 10% to about 80% by weight;

(c) vinylpyrrolidone-vinylacetate copolymers such as, e.g., random copolymers produced by a free-radical polymerization of the monomers in a molar ratio of from about 70/30 to about 30/70 and having weight average molecular weights of from about 14,000 to about 58,000;

(d) vinylpyrrolidone-dimethylaminoethylmethacrylate copolymers;

(e) vinylpyrrolidone-methacrylamidopropyl trimethylammonium chloride copolymers such as, e.g., those commercially available under the trade mark GAFQUAT®;

(f) vinylpyrrolidone-vinylcaprolactam-dimethylaminoethylmethacrylate terpolymers such as, e.g., those commercially available under the trade mark GAFFIX®;

(g) vinylpyrrolidone-styrene copolymers such as, e.g., those commercially available under the trade mark POLECTRON®; a specific example thereof is a graft emulsion copolymer of about 70% vinylpyrrolidone and about 30% styrene polymerized in the presence of an anionic surfactant; or

(h) vinylpyrrolidone-acrylic acid copolymers such as, e.g., those commercially available under the trade mark ACRYLIDONE® which are produced in the molecular weight range of from about 80,000 to about 250,000.

Accordingly, in some embodiments, those of skill in the art, using no more than knowledge available to the ordinary practitioner, the disclosure provided herein and routine experimentation, can select appropriate organic protective agents to preferentially produce silver nanocubes as compared with other nanostructures.

Ratios of Silver Compound to Organic Protective Agent

One of skill in the art will appreciate that the literature discussing selective production of one form of nanostructure as compared to another has suggested that the ratio between the concentrations of the organic protective agent (i.e. the concentration of the ‘OPA solution’) to the concentration of the silver solution can affect the types of nanostructures formed (Sun et al., Shape-Controlled Synthesis of Gold and Silver Nanoparticles, Science, 298: 2176-2179 (2002) at page 2177, col. 1). Indeed, it has been suggested that to produce a nanocube, the concentration of AgNO₃ in the final solution must be relative high (0.125-0.25M), and the molar ratio between PVP and AgNO₃ must be low (at ˜1.5) (See Wiley et al., Shape-Controlled Synthesis of Metal Nanostructures: The Case of Silver, Chem. Eur. J., 11: 454-463 (2005) at page 457, col. 1).

Applicants, however, have been able to produce nanocubes by addition of solid silver compound and/or solid organic protective agent directly to the reaction vessel (and/or reaction mixture). Consequently, these teachings do not directly correlate with the presently claimed subject matter.

Applicants believe that in general, nanocube production is favored when the molar ratio of organic protective agent to silver compound added to the reaction is greater than one. Nevertheless, with reference to the product solution, in some embodiments, the ratio of the molar concentration of the organic protective agent to the molar concentration of the silver (either as an ion or as metal) is in the range of about 10:1 to about 1.1:1. In some embodiments, the ratio of the molar concentration of the organic protective agent to the molar concentration of the silver (either as an ion or as metal) in the product solution is in the range of about 4:1 to about 1.2:1 or in the range of about 3:1 to about 1.25:1. In some embodiments, the ratio of the molar concentration of the organic protective agent to the molar concentration of the silver (either as an ion or as metal) in the product solution is in the range of about 2:1. Use of these higher concentrations conserves reagents and in general is more economically favorable, particularly for large scale production.

Accordingly, in some embodiments, using no more than the disclosure provided herein and routine experimentation, those of skill in the art can select appropriate ratios of organic protective agent to silver compound to preferentially produce silver nanocubes as compared with other nanostructures.

Optional Reactants

The literature has suggested that chloride ion and iron are common contaminates of ethylene glycol and the presence of these contaminates may influence the nature of the nanostructures formed (See: Wiley et al., Synthesis of Silver Nanostructures with Controlled Shapes and Properties, Acc. Chem. Res., 40: 1067-1076 (2007) at page 1068, col. 2). The iron contaminate may be in the form of Fe(II) and/or Fe(III). Applicants have determined that other halide ions such as bromide ion can be a contaminate of a polyol or other solvent that might be used.

It will be also understood that with respect to determining concentration, reference is made to the ‘product solution’ since the halide ion and/or iron may come from different sources. However, the product solution is formed from all the combined reactants and it is the concentration of halide ion and/or iron that is present in the reaction mixture that can affect the reaction. It is for this reason that concentrations of these compounds are expressed with reference to the product solution.

Because the halide ion and/or iron is often a contaminate of the polyol, a compound containing halide ion and/or iron (either as Fe(II) or Fe(III)) need not necessarily be added to the reaction as a reactant where a sufficient amount is introduced indirectly. However, if there is a deficiency, one may optionally combine at least one compound comprising halide ion and/or at least one compound comprising Fe(II) and/or Fe(III).

For this reason, it may be advisable to determine how much of these contaminates are present in the other reactants (e.g. the polyol) to be combined in the reaction prior to preparing the reactants for reaction. Many commercially available solvents and reagents come with a certificate of analysis which specifies amounts of iron and/or chloride ion as impurities. Based on this analysis a specified amount of at least one compound comprising halide ion and/or at least one compound comprising Fe(II) and/or Fe(III) can be added to adjust the concentrations to those specified above. Moreover, where the concentration of halide ion and/or iron would exceed the concentrations specified above because of contaminates, one could select different batches of reactants/solvents to thereby avoid this situation.

When compound comprising the halide ion is to be used, it can be combined with the other reactants in various ways. For example, it can be mixed by direct addition to the reaction vessel, which may contain polyol, it can be introduced with another reactant or it can be added directly to the reaction mixture (e.g. after all other reactants are added).

It has been reported that addition of Na₂S can assist in directing the production of nanocubes when practicing the polyol method (See: Skrabalak et al., Facile synthesis of Ag nanocubes and Au nanocages, Nature Protocols, 2(9): 2182-2190 (2007)). Thus, in some embodiments, Na₂S is included as a reactant in the practice of the claimed method(s) for producing nanocubes.

Thus, in some embodiments, using no more than the disclosure provided herein and routine experimentation, one of skill in the art can determine when optional halide ion, iron (as Fe(II) and/or Fe(III)) and/or Na₂S is desired as well as an appropriate mode for mixing with the other reactants and then select the appropriate reactants to preferentially produce silver nanocubes as compared with other nanostructures.

Supplemental Reducing Agents

In some embodiments it may be desirable to augment the reducing capacity of the polyol, for example, in order to reduce the required reaction time and/or the reaction temperature. The use of hydrogen as a supplemental reducing agent in the polyol process was suggested by Figlarz et al. (U.S. Pat. No. 4,539,041). Other non-limiting examples of reducing agents which may be employed in the polyol method include hydrazine and derivatives thereof, hydroxylamine and derivatives thereof, aldehydes such as, e.g., formaldehyde, hypophosphites, sulfites, tetrahydroborates (such as, e.g., the tetrahydroborates of Li, Na, K), LiAlH₄, polyhydroxybenzenes such as, e.g., hydroquinone, alkyl-substituted hydroquinones, catechols and pyrogallol; phenylenediamines and derivatives thereof; aminophenols and derivatives thereof; ascorbic acid and ascorbic acid ketals and other derivatives of ascorbic acid; 3-pyrazolidone and derivatives thereof; hydroxytetronic acid, hydroxytetronamide and derivatives thereof; bisnaphthols and derivatives thereof; sulfonamidophenols and derivatives thereof; and Li, Na and K metals (See: Published US Patent application No US 2007/0034052 A1 by Vanheusden et al.).

There are literature reports that poly(vinyl pyrrolidone) (PVP), which can be used as the organic protective agent, can be used as a reducing agent (See: Silvert et al., J. Mater. Chem., 7(2): 293-299 (1997) at page 296). As discussed herein, PVP can be used as a organic protecting agent. Consequently, in some embodiments, the organic protecting agent can be also act as a supplemental reducing agent.

Thus, in some embodiments, using no more than the disclosure provided herein and routine experimentation, one of skill in the art can determine when supplemental reducing agents are desired and then select the appropriate supplemental reducing agent(s) to preferentially produce silver nanocubes as compared with other nanostructures.

Reaction Temperature

The ‘reaction temperature’ is the temperature of the mixture once at least a portion of the polyol, the silver compound (or silver solution) and the organic protective agent (or OPA solution) have been combined (mixed). Numerous published methods for the polyol production of silver nanocubes employ a reaction temperature of 160° C. (See for example, Sun et al., Shape-Controlled Synthesis of Gold and Silver Nanoparticles, Science, 298: 2176-2179 (2002) at Note 25). In applicants hands, it seems that control of temperature is not so critical to the production of silver nanocubes for some embodiments.

For example, the reaction temperature can be less than or equal to 145° C. The reaction temperature can be (but is not necessarily) above ambient (room) temperature. In some embodiments, the reaction temperature is maintained from between 60° C. to 145° C. during the reaction. In some embodiments, the reaction temperature is maintained from between about 80° C. to about 135° C. In some embodiments, the reaction temperature is maintained from between about 100° C. to about 180° C. In some embodiments, the reaction temperature is maintained from between about 120° C. to about 175° C. In some embodiments, the reaction temperature is maintained from between about 125° C. to about 145° C. In some embodiments, the reaction temperature is maintained from between about 110° C. to about 190° C. In some embodiments, the reaction temperature is maintained from between about 150° C. to about 170° C.

The reaction temperature of the reaction mixture is, at least in part, determined by factors such as the heat source used to heat the reaction, the surface area of the reaction vessel that contacts the heat source, the boiling point of the solvent(s) included therein (e.g., the boiling point of at least the polyol), the thermal stability of the organic protecting agent, the reactivity of the silver compound and the polyol, and the temperature, volume and rate of addition of any reactants added as a solution.

Thus, in some embodiments, using no more than the disclosure provided herein and routine experimentation, one of skill in the art can select an appropriate reaction temperature to preferentially produce silver nanocubes as compared with other nanostructures.

Reaction Time

The reaction time is measured from the time that at least a portion of each of the reactants to be reacted are combined (i.e. there must be a mixture that contains at least a portion of each of the reactants that are to be reacted) and then extends through any time where a continued combining of the reactants occurs until the time when all reactants have been added to the reaction. The reaction time may also (but does not necessarily) include the time after all of the reactants have been combined during which nanostructures are produced. The reaction time may also (but does not necessarily) include the time after nanostructures are produced, the reaction is cooled, and until the process of separating the metal from the other components of the product solution (e.g. by decanting, filtration, precipitation, or centrifugation) is completed.

There is no limitation on the reaction time. It can be as short as 1-2 minutes (or shorter) or as long as a week (or longer). In general the reaction is complete when the silver metal has formed nanostructures. Although in some cases the reaction can be permitted to continue so that processes, such as Ostwald Ripening (See: Silvert et al., Preparation of colloidal silver dispersions by the polyol process, Part 2—Mechanism of particle formation; J. Mater. Chem. 7(2): 293-299 (1997) at the abstract and FIG. 14), can occur, this is not essential.

In practice, reaction time can be from about 3 minutes to about 24 hours. In some embodiments, reaction time can be from about 30 minutes to about 5 hours. In some embodiments, reaction time can be from about 1 to about 4 hours. In some embodiments, reaction time can be from about 2 minutes to about 1 hour. In some embodiments, reaction time can be from about 15 minutes to about 45 minutes. It is to be understood that these time frames are not limiting as the reactions time can also be extremely short or extremely long. The only limitation is that the reaction produces silver nanocubes. In some embodiments, the reaction time can be from about 2 minutes to several weeks, months or even years.

Thus, in some embodiments, using no more than the disclosure provided herein and routine experimentation, one of skill in the art can select an appropriate reaction time to preferentially produce silver nanocubes as compared with other nanostructures.

Mixing/Combining Reactants

According to embodiments of the invention, the reactants are combined (mixed) and reacted to thereby produce nanostructures, including nanocubes. The reactants can be combined as solutions, in solid form or as neat liquids provided that the silver compound and/or the OPA is combined as a solid (rather than as a solution). The order of addition of the reactants is not limiting as they can generally be combined in any manner and under any suitable conditions that produces nanocubes. For example, the silver compound can be combined with the organic protective agent (the OPA being combined as a solid or as an OPA solution) in a portion (or the entirety) of the polyol. The silver compound and the organic protective agent can, in some embodiments, be simultaneously added to the polyol. The silver compound and the organic protective agent can, in some embodiments, be added sequentially to the polyol. If an OPA solution is used, this addition can be dropwise or portionwise.

For solutions of OPA, portionwise addition is generally accomplished by pouring the reactant into another reactant or mixture with which it is being mixed over a short period of from about 1 to about 30 seconds. The period may be longer than 30 seconds and the exact time for pouring will depend on the volume to be added. Moreover, the portions may be the entirety of the reactant or may represent a portion, whereby the reactant is mixed in two or more portions which are combined serially or sequentially.

As discussed above, the silver compound and/or organic protective agent can be mixed in solid form. This addition can be continuous over a period of time or portionwise. Similar to the discussion pertaining to solutions, the order of addition of the solids is not limiting as the reactants can be combined in any way that produces silver nanocubes.

According to some embodiments of this invention, Na₂S can be used as a reactant. As with the other reactants, there is no firm limitation on the order of mixing so long as the reaction produces the desired nanostructure, such as silver nanocubes.

Thus, in some embodiments, using no more than the disclosure provided herein and routine experimentation, one of skill in the art can select appropriate forms of the reactants and order of their addition (as well as other conditions such as reactant concentration, reaction time and temperature) to preferentially produce silver nanocubes as compared with other nanostructures.

5. Various Embodiments of the Invention

It should be understood that the order of steps or order for performing certain actions (e.g. the addition of reactants) is immaterial so long as the present teachings remain operable or unless otherwise specified. Moreover, in some embodiments, two or more steps or actions can be conducted simultaneously so long as the present teachings remain operable or unless otherwise specified.

In some embodiments, this invention pertains to a method for the production of silver nanocubes. Said method comprises: a) combining as reactants: i) at least one polyol; ii) at least one silver compound capable of producing silver metal when reduced; and iii) at least one organic protective agent (OPA); wherein the silver compound is in solid form when combined with other reactants (rather than combined as a solution): and b) reacting the reactants to produce silver nanocubes. It is to be understood that the step of reacting involves selection of suitable reaction conditions such as reaction time, reaction temperature and reactant concentration. As described supra, optionally at least one compound comprising halide ion and/or at least one compound comprising Fe(II) and/or Fe(III) can be combined as a reactant. Optionally, Na₂S can be combined as a reactant. In some embodiments, the organic protective agent is in solid form (rather than in solution) when combined. In some embodiments, the organic protective agent is combined with the other reactants as a neat liquid.

Said combined reactants are reacted at a reaction temperature and under conditions that produce silver nanocubes. In some embodiments, the reaction produces a product solution comprising at least 10 weight percent (10 wgt %) silver nanocubes as compared with a weight percent of all other silver nanostructures in the product solution.

In some embodiments of the above described method, the weight percent of silver nanocubes, as compared with all other nanostructures in the product solution, is at least 15 weight percent (15 wgt %). In some embodiments, the weight percent of silver nanocubes, as compared with all other nanostructures in the product solution, is at least 20 weight percent (20 wgt %). In some embodiments, the weight percent of silver nanocubes, as compared with all other nanostructures in the product solution, is at least 25 weight percent (25 wgt %). In some embodiments, the weight percent of silver nanocubes, as compared with all other nanostructures in the product solution, is at least 30 weight percent (30 wgt %). In some embodiments, the weight percent of silver nanocubes, as compared with all other nanostructures in the product solution, is at least 40 weight percent (40 wgt %). In some embodiments, the weight percent of silver nanocubes, as compared with all other nanostructures in the product solution, is at least 50 weight percent (50 wgt %). In some embodiments, the weight percent of silver nanocubes, as compared with all other nanostructures in the product solution, is at least 60 weight percent (60 wgt %). In some embodiments, the weight percent of silver nanocubes, as compared with all other nanostructures in the product solution, is at least 80 weight percent (80 wgt %). In some embodiments, the weight percent of silver nanocubes, as compared with all other nanostructures in the product solution, is at least 90 weight percent (90 wgt %).

One of skill in the art will appreciate that the reaction can begin when at least a portion of all of the silver compound, the organic protective agent and the polyol have been combined and then continue while the remainder of the reactants are mixed and thereafter for a period of time (i.e. the reaction time). Thus, the description set forth above is not intended to be limited to situations where the reaction is not initiated until the entirety of each reactant has been mixed.

6. Examples

Aspects of the present teachings can be further understood in light of the following examples, which should not be construed as limiting the scope of the present teachings in any way.

Example 1 General Procedure for the Synthesis of Silver Nanocubes by Addition of Solid Silver Compound and Solid Organic Protective Agent

To a 20 ml vial at room temperature (RT) was added 8 ml of ethylene glycol (EG). The vial was then placed on a hotplate with its surface temperature set to around 200° C. After heating the EG for about 5 minutes (min) with stirring and with the vial cap on (it is estimated that the EG was still less than 100° C.), 0.03 g PVP (as a powder) was added. This process was performed in parallel for 12 vials. In addition to the PVP powder, to each of the 12 vials was added a different amount of Na₂S. Specifically, to each different vial was added a solution of Na₂S (3 mM in EG) in ten microliter increments in a range from 30 to 140 microliter (i.e. 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130 or 140, microliter to each different vial, respectively). Then, after another 5 min of continued heating, when the mixture was expected to be around 120-160° C., 0.024 g AgNO₃ powder was added to each vial and the cap was closed. In general the reaction was permitted to continue for 3 to 15 min this reaction time can be longer or shorter.

FIG. 1 is a transmission electron micrograph (TEM) images of the crude product obtained from the sample having 60 microliters of added Na₂S solution (i.e. 3 mM in EG). Based on the image in FIG. 1, it is estimated the crude sample contain at least 80 weight percent nanocubes. However, this sample is the only one of the 12 vials that contained predominately silver nanocubes as the reaction product.

7. References US Patent and Published Patent Applications

-   1. U.S. Pat. No. 4,539,041 to Figlarz et al. issued Sep. 3, 1985 -   2. US Published Patent Application No. US 2005/0056118 A1 to Xia et     al. published on Mar. 17, 2005 -   3. US Published Patent Application No. US 2006/0115536 A1 to Yacaman     et al. published on Jun. 1, 2006 -   4. US Published Patent Application No. US 2007/0034052 A1 to     Vanheusden et al. published on Feb. 15, 2007 -   5. US Published Patent Application No. US 2008/0003130 A1 to Xia et     al. published on Jan. 3, 2008

Other Patent Documents

-   1. U.S. Provisional Application No. 60/815,627, filed on Jun. 21,     2006 -   2. Chinese Published Patent Application No. 101220506, published on     Jul. 16, 2008

Scientific Publications

-   1. Fievet et al., Homogeneous and Heterogeneous Nucleations In The     Polyol Process For The Preparation of Micron And Submicron Size     Metal Particles, Solid State Ionics, 32-33: 198-205 (1989) -   2. Ducamp-Sanguesa et al., Synthesis and Characterization of Fine     and Monodisperse Silver Particles of Uniform Shape, Journal of Solid     State Chemistry, 100: 272-280 (1992) -   3. Silvert et al., Preparation of colloidal silver dispersions by     the polyol process, Part 1—Synthesis and characterization, J. Mater.     Chem., 6(4): 573-577 (1996) -   4. Silvert et al., Preparation of colloidal silver dispersions by     the polyol process, Part 2—Mechanism of particle formation, J.     Mater. Chem. 7(2): 293-299 (1997) -   5. Carotenuto et al., Preparation and characterization of nano-sized     Ag/PVP composites for optical applications, Eur. Phys. J. B., 16:     11-17 (2000) -   6. Sun et al., Shape-Controlled Synthesis of Gold and Silver     Nanoparticles, Science, 298: 2176-2179 (2002) -   7. Sun et al., Large-Scale Synthesis of Uniform Silver Nanowires     Through a Soft, Self-Seeding, Polyol Process, Adv. Mater. 14(11):     833-837 (2002) -   8. Sun et al., Polyol Synthesis of Uniform Silver Nanowires: A     Plausible Growth Mechanism and the Supporting Evidence, Nano     Letters, 3(7): 955-960 (2003) -   9. Wiley et al., Polyol Synthesis of Silver Nanoparticles: Use of     Chloride and Oxygen to Promote the Formation of Single-Crystal,     Truncated Cubes and Tetrahedrons, Nano Letters, 4(9): 1733-1739     (2004) -   10. Liu et al., Separation and Study of the Optical Properties of     Silver Nanocubes by Capillary Electrophoresis, Chemistry Letters,     33(7): 902-903 (2004) -   11. Yu et al., Controlled Synthesis of Monodisperse Silver Nanocubes     in Water, J. Am. Chem. Soc., 126: 13200-13201 (2004) -   12. Wiley et al., Polyol Synthesis of Silver Nanostructures: Control     of Product Morphology with Fe(II) or Fe(III) Species, Langmuir,     21(18): 8077-8080 (2005) -   13. Wiley et al., Shape-Controlled Synthesis of Metal     Nanostructures: The Case of Silver, Chem. Eur. J., 11: 454-463     (2005) -   14. Wiley et al., Maneuvering the Surface Plasmon Resonance of     Silver Nanostructures through Shape-Controlled Synthesis, J. Phys.     Chem. B., 110: 15666-15675 (2006) -   15. Wiley et al., Synthesis of Silver Nanostructures with Controlled     Shapes and Properties, Acc. Chem. Res., 40: 1067-1076 (2007) -   16. Skrabalak et al., Facile synthesis of Ag nanocubes and Au     nanocages, Nature Protocols, 2(9): 2182-2190 (2007) -   17. Fan et al., Epitaxial Growth of Heterogeneous Metal     Nanocrystals: From Gold Nano-octahedra to Palladium and Silver     Nanocubes, J. Am. Chem. Soc., 130: 6949-6951 (2008) -   18. Kundu et al., Polyelectrolyte mediated scalable synthesis of     highly stable silver nanocubes in less than a minute using microwave     irradiation, Nanotechnology, 19: 065604 (2008) -   19. Akhavan et al., Silver nanocube crystals on titanium nitride     buffer layer, J. Phys. D: Appl. Phys., 42: 105305 (2009)

While the present teachings are described in conjunction with various embodiments, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. Thus, the invention as contemplated by applicants extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.

Moreover, in the following claims it should be understood that the order of steps or order for performing certain actions (e.g. mixing of reactants) is immaterial so long as the present teachings remain operable. Unless expressly stated otherwise or where performing the steps of a claim in a certain order would be non-operative, the steps and/or substeps of the following claims can be executed in any order. Moreover, two or more steps or actions can be conducted simultaneously. 

1. A method for the production of silver nanocubes comprising: a) combining as reactants: i) at least one polyol; ii) at least one silver compound; and iii) at least one organic protective agent (OPA); wherein the silver compound is in solid form when combined with the other reactants: and b) reacting the reactants to produce silver nanocubes.
 2. The method of claim 1, wherein the reaction produces a product solution comprising at least 10 weight percent (10 wgt %) silver nanocubes as compared with a weight percent of all other silver nanostructures in the product solution.
 3. The method of claim 1, wherein the reaction produces a product solution comprising at least 50 weight percent (50 wgt %) silver nanocubes as compared with a weight percent of all other silver nanostructures in the product solution.
 4. The method of claim 1 further comprising combining at least one compound comprising halide ion as a reactant.
 5. The method of claim 1 further comprising combining at least one compound comprising Fe(II) and/or Fe(III) as a reactant.
 6. The method of claim 1 further comprising combining Na₂S as a reactant.
 7. The method of claim 1, wherein the organic protective agent (OPA) is in solid form when combined with the other reactants.
 8. The method of claim 1, wherein the organic protective agent is combined as a neat liquid with the other reagents.
 9. The method of claim 1, wherein at least one reactant is added portionwise over a period of time. 